The invention relates to a method for controlling the drive of a hydraulic press including the steps of providing a piston/cylinder unit having a piston connected to a press ram, a first cylinder chamber located below the piston, and a second cylinder chamber acted upon by the piston. The piston is acted upon by a hydraulic medium for displacing the ram press.
In hydraulic presses the press ram is connected to one or more piston/cylinder units. Important parameters of the hydraulic drive are the pressure and the delivery rate of the pressure medium. Both parameters can be adapted to the respective work procedure by means of appropriate pumps and control devices (power, pressure and stroke controllers). Depending on the type of drive a distinction is made between hydraulic presses with direct pump drive and hydraulic presses with accumulator drive (see, for example, Manfred Weck: Werkzeugmaschinen [Machine tools] Vol.1, pages 103 to 105). In the case of direct drive, a fixed-displacement pump or variable-displacement pump (hydraulic pump) driven by an electric motor acts on the main working cylinder which, for generating the press force, is acted upon with pressure medium on the large cylindrical piston surface and, for lifting the ram, is acted upon with pressure medium on the smaller ring surface of the piston of the piston/cylinder unit. The pump and the drive motor must always be designed for the maximum power requirement of the press. The high-pressure pump can in this case be designed as a variable-displacement pump or variable hydraulic pump in order to permit a stepless variation of the delivery rate and, consequently, of the ram speed. Accordingly, at low pressure a large quantity of fluid is delivered and as a result a great speed is imparted to the press tool, and vice versa. This is expedient for a quick stroke as rapid traverse and for a high force impact during the shaping procedure. However, a disadvantage is that the drive energy of the pump is constantly changing between zero and a maximum value. This results in considerable loads on the mains supply. A further disadvantage of the direct drive is that the potential energy of the ram and of the downwardly moved masses of the system remains unused, since the pressure medium flowing out from the annular cylinder during the downward movement of the piston is simply let off via a directional valve into a tank or oil container.
Another drive variant for hydraulic presses is the accumulator drive. Here, a fixed-delivery pump driven by an electric motor delivers first into a high-pressure accumulator, from which the working cylinder is then fed with the storage pressure via a proportional valve. However, this high storage pressure is available during the entire downward stroke movement and, thus, also already during the so-called rapid traverse, with a lower energy requirement per se. The actual shaping of the workpiece, with the high shaping force which is necessary for this and which is generated by the storage pressure, takes place only on the substantially smaller working stroke after the rapid traverse.
For the subsequent downward stroke, the accumulator must again be charged with the high energy or pressure which is utilized only to a small extent. This requires, for the charging pump, a correspondingly high electrical drive power.
Thus, in the case of the accumulator drive the overall energy balance is extremely unfavorable, and in practice degrees of utilization of approximately 20% to 30% are achieved.
Furthermore, in the accumulator drive of known presses the potential energy of the downwardly moving parts is not exploited, i.e. the hydraulic medium to be displaced is let off into oil containers. A recovery of the kinetic energy of the pressure medium flowing out from the piston/cylinder unit is not provided for.
The inflow of pressure medium through the hydraulic pump or hydraulic accumulator to the hydraulic cylinder of the press and the outflow of pressure medium from the annular cylinder chamber below the piston are controlled via multiway valves.